Compactly storing a firearm is a challenge. This is especially the case for firearms such as M-16/AR-15/AR-10 types which have mechanisms that use direct gas impingement. Many M-16 type weapons have a direct gas impingement system, and could benefit from being able to fold the stock; however, suitable mechanisms permitting a folding stock in a direct gas impingement system have not been developed.
Folding stocks are known. US Patent Application No. 201010307042 shows a modular firearm stock system which includes a foldable butt stock assembly. U.S. Pat. No. 7,966,761 shows an automatic or semiautomatic rifle with a folding stock. Similarly, US Patent Application No. 2011/0131857 shows an automatic or semiautomatic rifle with a folding stock which is the same as in U.S. Pat. No. 7,966,761. U.S. Pat. No. D636,834 shows simply a folding firearm stock without internal details. US Patent Application No. 2010/0212206, and U.S. Pat. Nos. 7,827,721 and 7,673,412 show folding stock assemblies that do not accommodate any mechanisms. U.S. Pat. No. 7,418,797 shows another simple foldable rifle stock which has an added adjustable cheek pad. These foldable butt stock assemblies simply function as foldable stocks and do not accommodate any actuation.
Firearms which incorporate hinges are also known. U.S. Pat. No. 7,802,392 teaches tactical firearm systems, and methods of manufacturing same, which includes a modular locking hinge having two parts that attach modular components; however, the hinge does not appear to be used for a foldable stock. U.S. Pat. No. 6,591,533 shows a locking hinge used with a folding shoulder rest for a paintball gun which basically is analogous to a rifle stock for a handgun.
Direct gas impingement systems in M-16 type weapons bleed combustion gas from the barrel and convey it through a hollow tube back to the weapon's upper receiver. The gas from the tube pushes rearward on the bolt carrier assembly that rides within the upper receiver. The rearward push on the bolt carrier assembly, and attached bolt, cause the assembly to move rearward. The rear of the bolt carrier assembly bears against a recoil buffer and associated spring which are contained within the buffer tube, buttstock, or a receiver extension. When the bolt travels rearward, it compresses the buffer spring; it is this force combined with the weight of the buffer which slows, and then stops, the bolt carrier assembly's rearward movement. The direction of movement then reverses, and the carrier assembly travels forward again, back into battery.